1. Field of the Invention
The present invention relates to a temperature sensor incorporating a thermistor element that is used to detect temperature in various locations, and which is suited for use as a high-temperature thermistor-type temperature sensor that detects catalyst temperature and exhaust system temperature in diesel engines and gasoline engines.
2. Description of the Related Art
In high-temperature thermistor-type temperature sensors that detect catalyst temperature and exhaust system temperature in diesel engines and gasoline engines of the prior art, a thermistor element equipped with a thermistor section and electrode wires (normally platinum wires) for acquisition of thermistor signals is housed in a cylindrical case. In order to prevent breakage of the platinum wires by absorbing mechanical stress due to engine vibrations and the temperature fluctuating from nearly xe2x88x9240xc2x0 C. to nearly 1000xc2x0 C., an opening in one end of the cylindrical case is filled with an insulating cushioning material such as alumina powder around the thermistor element to form a unit structure.
Moreover, in order to prevent entry of exhaust gas from the above opening in the cylindrical case as well as preventing the insulating cushioning material from spilling outside the case, the above-mentioned cylindrical case is inserted, from the end containing the above-mentioned opening, into a bottomed cylindrical metal tube. As a result, a double cylinder structure is formed that houses the above-mentioned unit, employing a form in which the above-mentioned opening is covered with the bottom of the metal tube.
However, accompanying higher engine speeds resulting from enhanced engine performance in recent years, due to the sensor holding structure at the site where the sensor is attached (catalyst or exhaust pipe, etc.) being reinforced, vibrations of greater magnitude are applied to the sensor at higher frequencies (e.g., 1 kHz or higher). Consequently, problems relating to the breakage of the platinum wires are becoming increasingly common, as indicated below, due to these intense, high-frequency vibrations.
The first problem is caused by the insulating cushioning material. In the above-mentioned structure of the prior art, spilling of insulating cushioning material to the outside is prevented by covering the opening in the cylindrical case with a metal tube, since the case is inserted into a metal tube. However, an extremely narrow gap (on the order of several tenths of a millimeter) neccessarily exists between the case and metal tube.
Here, when intense, high-frequency vibrations are applied to the sensor, the insulating cushioning material housed inside the case causes vibrations at the granular level due to the high frequency. These granules gradually break up and form tiny fragments that are able to enter the above-mentioned gap. Consequently, these tiny fragments spill out into the above-mentioned gap from the above-mentioned opening resulting in a loss of the thermistor element holding function and eventually leading to the risk of breakage of the electrode wires by vibration.
Another problem is caused by the material structure of the electrode wires themselves. Namely, although platinum wires are normally used for the electrode wires, according to a study conducted by the inventors of the present invention, during the course of manufacturing thermistor elements, the crystal grains of the platinum material become increasingly coarse in the firing process during which the platinum wires are molded by being embedded in a thermistor material (normally a semiconductor material) and firing (shrink-fitting) at 1300-1600xc2x0 C. Intense, high-frequency vibrations induce shifting at the grain boundary of the coarse crystals, thereby leading to the risk of grain boundary breakage of the platinum wires.
In this manner, the potential for breakage of electrode wires of thermistor elements will become even greater in the future due to the high-frequency vibrations accompanying higher engine speeds.
Therefore, in consideration of the above-mentioned problems of the prior art, the object of the present invention is to prevent breakage of the electrode wires caused by high-frequency vibrations in a thermistor-type temperature sensor equipped with a thermistor element having a thermistor section and electrode wires for acquiring thermistor signals.
The present invention was achieved by focusing on the two points consisting of (1) providing the thermistor element with a holding structure function so that insulating cushioning material does not come out of the case even if it breaks up, and (2) providing the platinum material used for the electrode wires with a structure so that the crystal grains do not become coarse even if exposed to high temperatures.
Namely, a first aspect of the present invention is characterized by being a thermistor-type temperature sensor equipped with a thermistor element equipped with a thermistor section and electrode wires extending from the thermistor section, a cylindrical electrically conducting first case having an opening in one end that houses the thermistor element, an insulating cushioning material of coagulated powder that is housed in the first case from the opening and insulates and holds the thermistor element in the first case, and a second case having a bottomed, or closed-ended, shape that houses and holds the first case so that the bottom covers the opening; wherein, a heat-resistant adhesive is juxtaposed between the first case and second case so as to at least seal the opening.
The above-mentioned xe2x80x9ccoagulated powderxe2x80x9d means a state of powder which is not sintered but can be shaved by a finger nail, and usually attained by heating a powder or slurry of an inorganic oxide to a temperature of 700xc2x0 C. to 1000xc2x0 C., preferably 800xc2x0 C. to 1000xc2x0 C.
This aspect of the present invention is based on the above-mentioned point (1). As a result of juxtaposing a heat-resistance adhesive between the above-mentioned first case and above-mentioned second case so as to at least seal the above-mentioned opening, even if the insulating cushioning material housed inside the first case is broken up and forms tiny fragments due to high-frequency vibrations, it can be prevented from spilling out from the opening into the gap between the first case and second case. Consequently, the thermistor element holding function can be maintained, and breakage of the electrode wires due to high-frequency vibrations can be. prevented. Furthermore, the heat resistance of the adhesive is preferably that which can withstand temperatures of, for example, 1000xc2x0 C. in consideration of exhaust temperature sensors and so forth.
The above-mentioned adhesive is preferably also disposed at the portion corresponding to the portion in which the insulating cushioning material of the first case is housed between the first and second case.
Although an insulating cushioning material of coagulated powder is normally filled into the first case from the opening in one end and contained in the form of a powder or slurry, in order to improve the ease of filling, a hole for venting air may also be provided in the portion of the first case in which the insulating cushioning material is contained in addition to the opening. Thereafter, the powder or slurry is heated to be coagulated. Thus, in this case, if the above-mentioned adhesive is disposed at the portion corresponding to the portion at which the insulating cushioning material of the first case is contained, the insulating cushioning material can be prevented from spilling out since the hole other than the opening can be sealed, thereby making it possible to prevent breakage of the electrode wires.
The temperature sensor of the above-mentioned first aspect includes a temperature sensor which has a so-called axial type of thermistor element. This temperature sensor is equipped with a wiring member for acquiring thermistor signals from the above-mentioned electrode wires to the outside which member comprises an electrically conductive outer tube electrically connected with the above-mentioned first case at the end opposite from the above-mentioned opening of the above-mentioned first case, and an electrically conductive core wire insulated and held within this outer tube;
the above-mentioned electrode wires are composed of a pair of electrode wires; and,
one of the electrode wires of the above-mentioned pair of electrode wires extends to the opposite side of the above-mentioned opening from the above-mentioned thermistor section and is electrically connected to the above-mentioned core wire, while the other electrode wire extends to the side of the above-mentioned opening and is electrically connected with the above-mentioned first case.
Moreover, on the basis of the above-mentioned second point (2), the inventors of the present invention earnestly conducted studies on an electrode wire material that does not break even if the insulating cushioning material breaks up, comes out of the case and the thermistor element is subjected to high-frequency vibrations. As a result, the inventors of the present invention invented the second and third aspects described below.
The second aspect of the present invention is a thermistor-type temperature sensor equipped with a thermistor element equipped with a thermistor section composed of a thermistor material and electrode wires for acquiring thermistor signals extending from this thermistor section, with the above-mentioned electrode wires being made of a dispersion-strengthened material having platinum or platinum alloy for its main component.
In this aspect of the present invention, the increasing coarseness of the crystal grains of the platinum material that occurs in the firing process of the thermistor element as described above is suppressed. Since high-frequency vibrations do not lead to breakage of the electrode wires even if they induce shifting of the grain boundary of the crystals, breakage of the above-mentioned electrode wires can be prevented.
It is preferable that the above-mentioned dispersion-strengthened material be a material in which 0.02 wt % or more of a metal oxide is added if the amount of platinum or platinum alloy is taken to be 100 wt %. Although the metal oxide has the function of suppressing increased coarseness of grains of platinum or platinum alloy, if the amount of said metal oxide is less than 0.02 wt %, it is unable to sufficiently suppress increasing coarseness of the grains of platinum or platinum alloy.
Moreover, it is preferable that said metal oxide be added at 2 wt % or less when the amount of platinum or platinum alloy is taken to be 100 wt % (namely, added within the range of 0.02-2 wt %). If the amount of metal oxide added is greater than 2 wt %, the ease of drawing of the electrode wire is dramatically impaired and the resistance of the electrode wire itself increases, thus making it difficult to adequately detect changes in the resistance of the thermistor element.
At least one type of substance selected from zirconia, yttrium, alumina and titania can be used for the above-mentioned metal oxide. Platinum alloy containing, alloyed with the platinum, at least one type of substance selected from rhodium, gold, tungsten and palladium can be used for the above-mentioned platinum alloy in order to improve the strength of the electrode wires.
Moreover, it is preferable that the crystal grain size in the direction of wire diameter of the electrode wires made of the above-mentioned dispersion-strengthened material be smaller than the wire diameter, and particularly preferably that grain size be one-half the wire diameter or less.
The effect of preventing wire breakage by the above-mentioned dispersion-strengthened material is sufficiently demonstrated even in electrode wires that are fixed to the thermistor section by shrink-fitting.
Use of the electrode wires made from a dispersion-strengthened material having for its main component platinum or platinum alloy as described above in the temperature sensor of the first aspect makes it possible to demonstrate an effect that combines the effects of both aspects.
A third aspect of the present invention is a thermistor-type temperature sensor equipped with a thermistor element equipped with a thermistor section comprising a thermistor material, and electrode wires extending from the thermistor section for acquiring thermistor signals; wherein, the electrode wires are composed of alloy wire of platinum and iridium. The use of this type of electrode wires makes the electrode wires themselves resistant to vibrations, thereby realizing prevention of wire breakage caused by high-frequency vibrations. Here, it is preferable that a platinum alloy be used for said alloy wire material in which the alloy composition consists of the addition of 1-60 wt % of iridium with the remainder consisting of platinum in consideration of wire drawing, thinning and breakage.
The above-mentioned electrode wires composed of an alloy wire material consisting of platinum and iridium sufficiently demonstrate the effect of preventing wire breakage even when the electrode wires are fixed to the above-mentioned thermistor section by shrink-fitting.
In addition, use of the electrode wires of the third aspect of the present invention as electrode wires of the first aspect of the present invention makes it possible to demonstrate an effect that combines the effects of both aspects.
The thermistor-type temperature sensor of the above-mentioned second and third aspects of the present invention may be equipped with a thermistor element equipped with a thermistor section made of a thermistor material and a pair of electrode wires extending in the same direction from the thermistor section for acquisition of thermistor signals,
a wiring member comprising a pair of electrically conductive core wires connected to the above-mentioned electrode wires and an outer tube that covers said core wires by means of insulating powder for acquiring the above-mentioned thermistor element signals to the outside, and
a cover that houses the above-mentioned thermistor element and welded to the outer tube of the above-mentioned wiring member;
wherein, the above-mentioned electrode wires are those of the second or third aspects of the present invention.